1. Field of the Invention
The present invention relates to noise reduction techniques.
2. Description of the Related Art
In addition to functions for capturing still images, some recent digital cameras have functions for capturing video, which involves recording audio signals. In such an image capturing apparatus, driving units operate when capturing video in order to drive a focus lens based on changes in shooting states, drive an aperture mechanism based on changes in the brightness of a subject, and so on. There is a problem in that the operating sounds of such driving units mix in with the recorded audio as noise. As such, a variety of conventional techniques have been disclosed for driving noise reduction processing.
Japanese Patent Laid-Open No. 2008-077707 relates to the removal of driving noise produced by a storage device in a video camera. According to Japanese Patent Laid-Open No. 2008-077707, audio that does not contain noise is predicted based on an audio signal occurring before and after a period in which driving noise is present, and the data of the driving noise-containing period is replaced with the predicted data. This technique makes it possible to record audio that does not contain driving noise even if a high level of driving noise is present.
Although unrelated to driving noise reduction, Japanese Patent Laid-Open No. 06-244808 discloses performing interpolation through audio prediction. Japanese Patent Laid-Open No. 06-244808 discloses a technique in which when an error has occurred during audio decoding, the signal of the period where the error occurred is predicted from a past audio signal and the signals are then interpolated.
A technique such as that described above, where audio that does not contain noise is predicted from audio data occurring before and after a noise-containing period, is comparatively effective. However, in the case where the noise-containing period is long, the signal waveform of the sound may change within that period. For example, in the case where the noise-containing period lasts 100 milliseconds, it is entirely possible for the sound “ah” to occur toward the beginning of the period but then change to “oh” by the end of the period. If a predicted signal is then created from the parts of signal before and after the noise-containing period and the two are combined to interpolate the audio as in Japanese Patent Laid-Open No. 2008-077707, the “ah” and “oh” sound will be mixed together, resulting in unnatural audio being generated. Japanese Patent Laid-Open No. 06-244808, meanwhile, interpolates the audio for long noise-containing periods by predicting the audio while gradually attenuating the predicted waveform, but if the same audio is present before and after the noise-containing period, the resulting audio will have a reduced sound pressure and will thus sound unnatural.